This invention relates to a method of carrying out conversion between first and second signal successions from one to the other in an adaptive predictive pulse code modulation (ADPCM) system and to an adaptive predictive encoder and decoder in which the method is used.
In ICASSP 82 Proceedings, Volume 2 (May 1982), pages 960-963, T. Nishitani et al describe under the title of "A 32 kb/s Toll Quality ADPCM Codec using a Single Chip Signal Processor," two types of adaptive predictive conversion systems each of which comprises an encoder and a decoder and which carries out adaptive quantization and adaptive prediction in the encoder and the decoder. Different algorithms are used in both of the conversion systems on the adaptive prediction. The adaptive prediction successively proceeds in both systems by the use of an adaptive predictive portion having a feedback loop and a plurality of prediction coefficients adaptively varied through the feedback loop.
It is desirable that an internal state of the encoder quickly becomes equal to, namely, coincides with that of the decoder. However, existence of the feedback loop makes the quick coincidence difficult once incoincidence takes place between the internal states of the encoder and the decoder for some reason or another. In other words, an incoincident duration becomes long with each of the systems.
Interaction between the adaptive quantization and the adaptive prediction makes the incoincident duration longer and longer with an increase of the number of the prediction coefficients.
Exact coincidence between the internal states of the encoder and the decoder is strictly required in a conversion system of multiple stages wherein ADPCM encoding and decoding are successively repeated to transmit a nonlinear PCM signal based on CCITT recommendation G 712. Although such a multiple-stage conversion system is described by H. Murakami in Proceedings of 1979 ISCAS, Pages 969-970, under the title of "A Low Noise ADPCM-LogPCM Code Converter," no description is directed towards quick coincidence of internal states between an ADPCM encoder and an ADPCM decoder of each stage. As a result, each code is gradually degraded at every stage.